Today I Learned Notes to self about software development

    Networking: Subnetting

    #networking
    03 Jan 2026

    Notes from Subnetting Mastery playlist.

    Seven attributes of subnetting

    1. Network ID - First IP address on each sub-net
    2. Broadcast IP - Last IP address on each sub-net
    3. First Host IP - IP address after the Network ID
    4. Last Host IP - IP address before the Broadcast ID
    5. Next Network - Network ID of the next Sub-Network
    6. Number of IP Addresses (available on sub-network)
    7. CIDR/Subnet Mask
    • Network ID and Broadcast IP are reserved and can’t be given to devices

    CIDR notation

    • identifies size of particular subnet (alternative to subnet mask)
    • examples: /24, /25, …, /27

    CIDR - Subnet Mask

    • /24 - 255.255.255.0
    • /25 - 255.255.255.128

    • /27 - 255.255.255.224

    • Where does the CIDR number come from?

    Draw Cheatsheet

    1. Start with 1, double until 128 (right to left)
    2. Subtract top from from 256
    3. Start with /32, decrement by 1 (right to left)

    128 64 32 16 8 4 2 1 (group size) 128 192 224 240 248 252 254 255 (Subnet mask) /25 /26 /27 /28 /29 /30 /31 /32 (CIDR)

    How to use:

    • Convert to from CIDR to Subnet
    • Use last octet from target IP and group size to find which block the target IP is in
      • Start from 0 and increment by group size until you exceed target IP
    • Number BEFORE target is Network ID
    • Number AFTER target is Next Network
    • IP BEFORE Next Network is Broadcast
    • IP After Network ID is FIrst Host
    • IP BEFOE Broadcast IP is Last Host
    • Group size is number of IPs

    Example:

    find 7 attributes for the following IP:

    10.1.1.37 /29
    • convert CIDR to subnet
      • .248
    • use last octet and group size to find which block the target IP is in (must exceed target IP)
    • number BEFORE target is Network ID
    • number AFTER target is Next Network
    • IP BEFORE Next Network is Broadcast
    • IP After Network ID is FIrst Host
    • IP BEFOE Broadcast IP is Last Host

    • Group size is number of IPs

      • 0, 8, 16, 24, 32, 40
      • host? # is between 32 and 40
      • means network ID is 10.1.1.32
      • means broadcast IP is 10.1.1.39
      • means First Host IP is 10.1.1.33
      • means Last Host IP is 10.1.1.38
      • means Next Network ID is 10.1.1.40
      • means Number of IPs is 8 (6 usable)

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